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4WD vs. AWD. What's the Difference?

404 points by hackuser 10y ago

bitL 10y ago
Great! Finally somebody explained this to me, I thought it was just a marketing term related to different patents of similar ideas. I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I thought they were the same, just different names, so I argued with the ranger that I do in fact have 4WD, which is in my case just called AWD and in other cases 4x4 as well. Well, he was still pretty upset but the "I am a European and we have different cars" finally disarmed him...
- japhyr 10y ago
  Did they make you turn around? Did your car do fine, or did it get a bit hairy?
  - bitL 10y ago
    He actually stopped me after climbing up back to the viewing platform. The car was some US-made SUV with high clearance (don't remember if it was Ford or GM), just there was a sign AWD in the back (I specifically asked for a 4x4 rental vehicle). The road was fine, I think the issue might have been a few streams you had to cross at the bottom of the valley, but SUV had no problem with them. Ranger wanted to fine me but after arguing about 4WD, AWD, 4x4 he dropped it, still a bit angry.
- blackguardx 10y ago
  Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.
  - ghaff 10y ago
    The Waipio Valley road, as I understand it, is mostly paved but extremely steep and narrow/twisty. So the 4x4 requirement is probably more about being able to control speed without burning up brakes and dealing with often wet conditions. However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of saying that, if you just have a random rental car and aren't used to driving very rough and often narrow/exposed roads, this is probably not a good idea.
    - dsr_ 10y ago
      And in particular, the Hawaiian rental car counters will tell you when you rent which roads your car is not allowed to go on.
      - rconti 10y ago
        I've always wondered why this matters. I've got car insurance, I've got additional rental coverage on my credit card, and there are plenty of other vendors from whom to rent cars if you want to blacklist me.
        Drdrdrq 10y ago
        Well, they still have trouble with you if you get stuck or wreck the car, so it is easier for them to just list the "forbidden" roads. Not sure, but your insurance might be void if you crash the car where you were not supposed to drive.
        mikeash 10y ago
        They probably don't want you taking a road you shouldn't, getting hurt, and then suing them because they didn't warn you about the dangers. Or they get tired of having all their cars smashed up. Insurance covers it, but it's still annoying.
        bitL 10y ago
        Having violated this all the time I've been to Hawai'i I can understand why do they do that - for example, if you continue on the road to Hāna/Maui beyond, down to the southern side of Haleakalā the road becomes really dodgy, later turns into gravel and there is no water/gas for xy miles available, so for casual American tourists it might be pretty scary. Also, driving the north-western part of Maui is on a very narrow road over cliffs and fallen rocks (not as bad as parts of Tenerife though). I actually had an encounter on a pretty tall cliff where I was driving Subaru Forrester and some US female celebrity named Kim was driving the opposite way in even a larger car; we drove past each other with smiles in about 1mph to avoid collision and she must have been right on the edge with her car while I was almost scratching the right side rocks. Even Saddle Road between Maunas on the Big Island could be pretty demanding to casual tourists in fog and really slippery when descending down to Hilo in strong rain, which is like always (saw a crash once). There are also some scary roads on Kaua'i and O'ahu. In US it's so much easier to get a driving license than in EU, so your casual drivers are not used to fast and safe driving like in Germany, many off-road segments like in Spain or navigating chaotic situations as in 3rd world countries, and your society is generally on the safe side (even excessively so).
- wil421 10y ago
  AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road. 4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so. I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road". Cars don't have proper ground clearance.
  - waterphone 10y ago
    > The downside is you shouldn't use it over 10mph or so. That's not strictly true, and varies between implementations/transfer case designs. In Jeep Wranglers, for example, which have both high and low gearing options in 4WD, the owner's manual will indicate that you can drive at any speed (on loose surfaces such as sand or dirt) in 4HI and 0–25 MPH in 4LO.
    - wil421 10y ago
      You right that's why I said 10 or so. Try taking a turn at 25 in 4LO. I'm not sure of the use case for 25 in LO. My Grand Cherokee can operate the same way but I doubt I would go into LO at those speeds. It also will switch to 4HI when it detects wheel slip. A friend of mine had a old truck that would randomly switch to 4LO on the highway. It's pretty scary driving 60 and having 4LO switch on.
      - brianwawok 10y ago
        In dirt taking the turn at 25 is fine. The inside wheel will just spin a little.
        refurb 10y ago
        Outside wheel wants to travel more than the drivetrain so it would slip, not spin correct? The inside wheel would spin if the outside doesn't slip.
- to3m 10y ago
  This distinction isn't really understood in the UK either, at least not as far as I've ever seen. If your car drives all 4 wheels, it has 4 wheel drive - no, seriously! - and you just have to know what its limitations might be. AWD vs 4WD seems to be one of those distinctions that Americans have, like drink vs beverage, that British English lacks completely.
japhyr 10y ago
Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. This provides great traction, but a vehicle locked in 4WD cannot safely be operated on dry pavement because its front and rear axles are forced to rotate at the same speeds." I remember being scared when I bought my first 4wd vehicle, a '91 Toyota pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and I was worried about going from snowy roads to dry pavement. When I moved on to an Outback (AWD, not 4wd) I wondered why I didn't have to worry about transitioning between slippery roads and dry pavement. Now I understand. There's also a great video from 1937(!) that shows why we need differentials, and how they work. Here's a direct link to the video if you're going to skip the article: https://www.youtube.com/watch?v=yYAw79386WI https://www.youtube.com/watch?v=yYAw79386WI
- neom 10y ago
  woooow! Re: youtube video - Go to market in 1930s was actually how to go to market. When you think that Chevrolet really got going in 1920, 17 years later their marketing was super on point (here we are still watching it today, content marketing at it's best).
- sunstone 10y ago
  And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes more sense.
  - Mikeb85 10y ago
    Well both tactics work in different scenarios. When power goes to the slipping wheels, it helps keep the car drive strait. For example, in the rain or light snow. If one wheel slips, then suddenly power goes to the wheel with traction, you'll veer off-course. When it goes to the wheels with traction, it helps get the car un-stuck. This is usually what you want when you're doing serious off-roading, stuck in snow, etc... Also, in this scenario you're usually going very slowly/not moving, so the sudden change of direction won't be a problem. Modern AWD does a bit of both - it'll send power to wheels with traction, but will also compensate by sending power to a wheel on the other side of the car. For example, if there's no traction on your left front tire, it'll send power to your right front tire and left rear tire, and reduce power on your right rear tire to keep you going strait.
dghughes 10y ago
>Torque goes to whichever of the four wheels has the least grip. That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere. In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4WD and AWD?
- singlow 10y ago
  He is talking about what happens when there is no lock. He goes on the clarify what happens when there are locks or viscous couplings.
- rwmurrayVT 10y ago
  >To counteract this, the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. The differential contains a nifty little clutch. When the vehicle detects no traction, it engages and sends the torque to the correct location. Otherwise you'd end up in a situation similar to engaged cruise control with no traction. Limited slip differentials and locked differentials cause much confusion.
- johngalt 10y ago
  Mechanically a drivetrain has no means of differentiating between a tire that is slipping vs a tire at that's on the outside of a turn. A 4wd car with three open differentials will absolutely send all of the engine power to one slipping wheel. The author is describing the tricks used to get around that fact. Namely locking differentials or limited slip differentials. The end result is a fraction of the torque makes it to other Wheels depending on the design of the drivetrain. The author spent a lot of time on locking differentials when those are rare. Limited slip differentials are more common. Most awd cars will have a LSD in the center diff. There are torsen diffs which do what you describe but they are rare.
  - kinofcain 10y ago
    Even a torsen diff, or similar helical/gear type limited slip diffs are "torque multiplying", they need a load on one wheel in order to send the "multiplied" load to the wheel with traction. Modern stability/traction systems equipped with helical diffs will apply the brake to the "free" wheel in order to provide a load to the mechanical diff. You can also apply the brakes yourself if you have a torsen diff but no electronics. A clutch-based diff or some other type of locking diff would otherwise be required to solve the "wheel in the air" problem without applying an artificial load to the free wheel.
deleted 10y ago
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- deleted 10y ago
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spacerace 10y ago
Jesus what a SHIT article It gets so many things wrong I don't even know where to start. Why is this garbage spam on Hacker News?
- rwmurrayVT 10y ago
  No it doesn't. I think you're just confused on fundamental mechanics.
  - spacerace 10y ago
    That's rich. The lack of understanding of the fundamental mechanics is exactly what makes this article shit and your comment along with it. But do go ahead a school me on this PLEASE.
    - phpnode 10y ago
      We're all here to learn, if you've got some insight into what the article got wrong then why don't you school us?
    - spacerace 10y ago
      To user: phpnode that replied to me. I can't reply directly to you but that's a fair enough point. I was avoiding doing so before because I didn't (and still don't) think this article is even deserving of being picked apart. I would be happy to do so for the sole benefit of users here though. Just give me a little time to get back from doing some errands and I promise I'll clarify my grievances with the information in the article.
      - phpnode 10y ago
        Still waiting for you to make good on this promise :)
    - sctb 10y ago
      These kinds of comments are not OK on Hacker News. Please stop. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
kinofcain 10y ago
There's a lot of blurring and different labeling of 4WD systems based on the market segment. Generally it's more helpful to think of 4WD/AWD as fitting into three categories: 1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be welded and they'll turn at the same speed. 2: A system with a center differential, in which both axles are driven but may turn at different speeds (Almost always labeled AWD). The differential may allow full or progressive locking and/or torque vectoring. It's rare to find one of these that can fully lock (Subaru WRX STI is the only one I can think of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or that can partially lock in response to slipping of one axle (often with the brand name Torsen). These are almost always found on passenger cars, and with few exceptions they're found on cars with longitudinally mounted engines (Mitsubishi EVO being the only transverse layout car with a center diff I can think of), that means they're usually found on higher end cars that are based on front-engine rear-wheel drive models. 3: A system with a power-take-off unit (often brand name Haldex), in which one axle is driven and a clutch pack can partially or fully engage the other axle. (this is often labeled AWD when put on a passenger car, and 4WD when placed on a "truck") The axles can operate at different speeds but can also fully lock in the way a transfer case can. Typically you find these on "AWD" cars that are based on transverse-mounted front-drive models as it's easier to add such a system and get partial engagement of the rear axle, but you'll also find them on performance AWD applications based on rear-drive mid/rear-engine models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford Raptor. These tend to operate more like an automatic transfer case, as the only way to partially engage the secondary axle is to slip the clutch, which you don't want to do 100% of the time. (Ford actually calls this system a transfer case on the Raptor). You'll see all sorts of spurious claims about systems "sending 100% of the torque to the rear axle" (often just means that the system can fully lock, and 100% torque to the rears means the fronts are in the air). You'll also see things like "rear biased", which sometimes means you have a center differential that overdrives the rear axle by default, which is great, but is often misleadingly applied to PTO/Haldex systems where the rears are also overdriven, but only during the rare times when the system is engaged. Basically if you're looking for traction in low speed settings where you might not have grip at all on one axle, like offroading or being stuck in the snow, you want as many of your axles locked as possible, which technically you can get with any of these systems, but is usually seen on transfer cases or haldex/PTO systems. If you're driving quickly on pavement with some traction, you usually want something more like a center differential, preferably one that can partially or entirely lock when you lose some grip at one end.
ebbv 10y ago
One thing that this article only touches on briefly is that more and more AWD systems today are just open differentials plus the TCS instead of using more expensive and heavier limited slip differentials. These modern systems are lighter and cheaper but they have disadvantages; not least of which is that using the brakes to force power to move around wastes torque, and it's only going to be as good as the sensors. If the ABS/TCS system has any issues your AWD goes out too. I've owned cars with every type of AWD system (Audi, Subaru and currently a Honda) and day to day they all work just as well. Given that, I suppose the manufacturers are ultimately right to go with the cheaper and weight saving system.
- dsfyu404ed 10y ago
  The brake based system is great if the traction control is programmed for "get me up this snowbank" and not "prevent me from being an idiot" There was an episode of Dirt Every Day where Fred and co. took a Jeep Renegade(?), or whichever the small one is, off-roading there were some great scenes of the traction control in action. Basically by spinning the dial to rocks the traction control knows to use ABS to approximate how one would work the brakes with a torsen.
- chiph 10y ago
  The Honda VTM-4 system is quite good in it's intended operating environment - which is operation during winter weather. It uses electromagnetic clutches in the rear differential housing to direct torque to the rear wheels with left/right independence. This gives you the effect of a locking differential, up to the friction limit of the clutch material. There's no center differential in the system, and the front differential is open (but Honda includes a TCS, which is cheap because once you have the ABS sensors & pump, it's just software). Even during good weather on dry roads, the system has benefits by engaging the clutches when accelerating from a stop, so all four wheels are driven when you need to get going. The clutches are disengaged after about 20 mph so you get the fuel economy of a FWD vehicle.
mschuster91 10y ago
I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trailer in trucks. And a fuel-electric drivetrain could easily plug in a battery or supercaps for braking energy recuperation, and the fuel engine could always run at peak efficiency/exhaust gas condition...
- kinofcain 10y ago
  The diesel-electric/hydraulic systems are used in situations where the gearing ratio would have to be absurdly high, they're not used because they're efficient or because they have good torque vectoring capabilities. The electric motor can produce nearly 100% of torque at zero RPM, which helps locomotives get several million pounds of train moving. In your car, truck, or even semi, first gear plus some clutch/torque converter slippage is plenty of mechanical advantage to get going, and once you're underway, a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor. There's also the matter of power distribution, if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.
  - 4ad 10y ago
    > a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor. I doubt it, generators and motors operate at 99% efficiency. Do you have any reference? > if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel. That assumes that engines are so small that they operate at 100% under normal conditions. This doesn't have to be the case, electric engines are so small, you can have bigger-sized engines where one single engine can use all the power. The nature of electric engine is such that the loss of efficiency by doing this is minimal. Plus you need to do this only fractions of the time, so the engine doesn't have to be designed to operate in high power mode at full duty cycle.
Animats 10y ago
There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range. It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well documented, but Tesla cars aren't intended for off-roading. The most advanced systems are seen on locomotives. Modern heavy locomotives (GE Evolution series) use three-phase AC synchronous motors driven by IGBT inverters from power from a Diesel generator. Each axle has its own motor. All axles are normally locked together electronically by the control software, so no wheel can slip ahead of the others. Locomotives can be cabled together so that the axles on multiple locomotives synchronize. Now that's all-wheel drive. This is a huge win when starting a heavy freight train. Locomotive axles that are slipping provide little pulling power and damage the tires and rails.
- hackuser 10y ago
  > Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Is that different than the traction control used by other automakers? > Tesla's all wheel drive system has a separate motor for the front and rear wheels. That is very cool. And it sounds much more complex - I wonder if the complexity is worth the benefit.
  - CarVac 10y ago
    Separate motors for front and rear wheels is far less complex than adding more moving parts. For AWD, you need a driveshaft and a differential. The differential has a lot of parts, there are a lot of bearings that sap power, and it adds a lot of weight. Using separate electric motors for front and rear instead increases the available power, enables unlimited torque ratios from all-forward to all-rear, and has many fewer moving parts.
    - 10y ago
dsfyu404ed 10y ago
The performance of either system is completely dominated by the traction aid at the axles. The article is otherwise good because most AWD vs 4WD stuff completely igores the axles (because it's a heck of a lot to explain if you really want to compare the various systems) but kinda misses the main point. Open, bad traction control (cuts throttle, applies a little brake), good traction control (all brake no throttle), the many varieties of LSDs, automatic lockers, manual lockers, Lincoln lockers and spools all have a huge affect on traction. Lincoln lock the diffs of any AWD crossover, screw with some wiring so it's full time and you'll have something that can out wheel a $50k Jeep but trades off a tiny bit of grocery getting capability.
- rsync 10y ago
  Thank you for introducing me to the concept of lincoln-locking a differential. Appreciated.
ynniv 10y ago
Of course, this only matters when you are driving four wheels with one engine. Now that we're going electric you can have one engine per wheel, or one front engine and one back engine and use ABS across single axles. It's kind of amazing how much progress we have made with simple internal combustion engines.
schwede 10y ago
The video from the 30s was fascinating. It gave a perfect progression of complexity while explaining the concept of a differential. I'd like to see more of those kind of videos (from now our from clear back then).
- auganov 10y ago
  You might like this one https://www.youtube.com/watch?v=mBz30JhtKkg https://www.youtube.com/watch?v=mBz30JhtKkg The channel also has some more.
- LyndsySimon 10y ago
  You might like Sheaffer's "The Twenty-Six Old Characters", c. 1947. It touches a bit on the progression of language, but it's really about writing instruments. It culminates in explaining Sheaffer's "Snorkel" mechanism, which was probably the most novel and complex pen system ever designed. I have several and love them :) https://youtu.be/1xUDehNvbrE https://youtu.be/1xUDehNvbrE
ledriveby 10y ago
Subaru's higher end AWD system has separate output shafts from the engine for the front and the rear, with electronically-controlled clutches modifying the torque. Default is 60/40 front biased, but it can send 100% of the torque to either end of the car. Excellent performance with no manual mode switching needed, also manages good fuel economy at the same time. Best system in the business.
- bluedino 10y ago
  However, if you don't have locking differentials you can still get into the situation where one wheel slips and the wheel that grips doesn't get any power - as shown by the Subaru getting stuck on the hill in this video: https://youtu.be/hrdb_UVTa20?t=543 https://youtu.be/hrdb_UVTa20?t=543
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  - post_break 10y ago
    There is a trick that seems idiotic but if you have a mechanical hand brake you can stop the wheel from free spinning. I'm not sure how well it would work for a front wheel, maybe you could hold the brake slightly while on the throttle. Again, seems ass backwards but if you think about what you're doing you're basically "locking" the open diff.
- abstrakraft 10y ago
  Not quite. All AWD Subarus (i.e. except the BRZ) have one output shaft out of the engine, 3 out of the transmission/transaxle (2 to the front tires, 1 to the rear diff), and 2 out of the rear diff. Torque split varies between models and years with most near the 60/40 you quote, but the only one (in the US, anyway) that can send 100% torque to either end is the STI with DCCD on full lock.
- pps43 10y ago
  To add to confusion, Subaru sells several very different systems and calls all of them the same "all time all-wheel drive". There is viscous coupling center differential that goes with manual transmissions and nearly always splits torque 50/50 between front and rear, computer-controlled limited-slip clutch pack that comes with most automatics and changes torque split with speed, and some newer systems that do even more.
kilovoltaire 10y ago
Wow, early cars connected the engine to only one wheel, to completely avoid issues of differential. (According to the cool old video linked in the article.)
- mod 10y ago
  My cousin had a firebird, I think a '67, that (seemingly?) had 1wd. He could do donuts that left one trail. Not sure if by design or some kind of failure...
giardini 10y ago
4WD/AWD are almost never necessary in USA and Europe today, to the extent that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk (for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in bad weather. But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and maintenance are significantly higher: often you must "fiddle" with them, woe to you should you damage them due to bad judgement. They reduce mileage, increase car weight, and reduce reliability. I prefer a RWD car for most driving, since it handles better and I'm never climbing bare hills or plunging through swamps, but I was raised on oversteering go-karts. "All Wheel Drive Does Not Make You Safer": http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-does-not-make-you-safer/ http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-do... "The Myth of the All-Powerful All-Wheel Drive:" http://www.popularmechanics.com/cars/a3091/the-myth-of-the-all-powerful-all-wheel-drive-15202862/ http://www.popularmechanics.com/cars/a3091/the-myth-of-the-a... 4WD/AWD safety - Is 4WD/AWD safer on snow and ice? http://www.4x4abc.com/jeep101/safe.html http://www.4x4abc.com/jeep101/safe.html
- landryraccoon 10y ago
  In California you are legally required to either have chains equipped or a 4WD vehicle with snow tires under R1 and R2 snow conditions: http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm It doesn't matter what the studies say, 4WD is legally recognized as safer than 2WD in California.
  - giardini 10y ago
    AFAICT you're required to have the chains available, so they're usually in the trunk, unless you're in a situation where they're required. Of course if you buy chains you should immediately put them on your vehicle, if only to ensure that later, when you need them, they will fit. Once you've done that, then you can put them in the trunk. I disagree with your interpretation of the law: dot.ca.gov simply state what they require and say nothing about which is superior. And note that 4WD/FWD vehicles _also_ must carry chains in the R2 & R3 areas (from the URL you posted http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm): "(NOTE: Four wheel/all wheel drive vehicles must carry traction devices in chain control areas.)" Unfortunately for an AWD/4WD vehicle, following this provision costs twice as much as for a FWD car and will take twice the time to install (not an insignificant factor in freezing rain) and, should you fail to install on both front and rear properly, or should they suffer mechanical failure, may put one in a bad situation. Sometimes it's better to do less.
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tzs 10y ago
The problem of the outer wheel going farther during a turn also occurs with trains, and trains do not have differentials to let the wheels have different angular velocity. On a train, the inner and outer wheels have the same angular velocity regardless of turns. How trains accomplish this is interesting: http://jalopnik.com/5820296/how-trains-make-turns-without-differentials http://jalopnik.com/5820296/how-trains-make-turns-without-di... I'm linking to Jalopnik, which embeds Richard Feynman's video on train turning, rather than directly to the video because there are some interesting comments on the site. In particular, see the comment from railroad employee "Gonemad" about how they have a system that automatically greases the wheels before turns, which seems rather counterintuitive at first.
justinph 10y ago
In real life, in most situations, ground clearance, good tires, traction control and ABS make more difference than driving all four wheels, most of the time. Personally, I'm more concerned with stopping than starting, and that's where ABS and good tires matter and what kind of transmission you have doesn't. This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with different vehicles on rollers: https://www.youtube.com/watch?v=9cuZYTQLfA0 https://www.youtube.com/watch?v=9cuZYTQLfA0 I spent a lot of time looking at this several years ago when my family was planning on taking a vacation to the Outer Banks of North Carolina where you have to drive on the sand. We ended up renting a Ford Explorer that had a "sand mode" in it's electronically controlled awd setup. Worked great.
- analog31 10y ago
  Something I've learned is that snow tires make a big difference during the winter, and I've read that they make a bigger difference than AWD.
  - justinph 10y ago
    Absolutely. Have owned a Prius and a Subaru. The Prius with good snow tires was phenomenal in the snow, better than the Subaru with all-seasons.
  - paganel 10y ago
    Have been a driver for only 2 years but I can confirm that. I live in Eastern Europe where we tend to have at least one or two snow storms per winter, and, indeed, having winter tires and ABS makes a ton of difference, even on a 1.4-liter FWD vehicle like mine. And, even more important, you have to keep a level head when you see that there's snow on the road and not try to do anything crazy, like making sudden left or right wheel movements. Also try to keep a safe distance from the vehicle in front of you, and, before you venture on a trip outside of the city, just check the weather forecasts: if there's a "red code" storm announced you'd better postpone your journey, only a tank can get you out from something like this: https://www.youtube.com/watch?v=Frf2lV77fl0 https://www.youtube.com/watch?v=Frf2lV77fl0
examancer 10y ago
Kind of an odd post to see on hacker news. As somewhat of a car buff my take away is that this article gives an adequate high level explanation but leaves out a lot of useful details, especially to anyone evaluating 4WD/AWD vehicles, like the tremendous differences between modern AWD systems.
elihu 10y ago
It'll be interesting to see what happens when off-road vehicles start becoming electrified. Separate front/rear motors eliminate the need for that middle differential or transfer case, and if we eventually transition to a separate motor for each wheel, we'll be rid of all of the differentials and have much better ground clearance as well as better traction. For now, I suppose electric trucks/jeeps are more affected by "range anxiety" than daily-driver sedans, since people tend to imagine themselves driving to some remote location when they buy them. But as technology improves, I would love to see something like a modern reincarnation of a 1960's era CJ-5 (but with better safety features) -- simple, easy to modify, nothing that doesn't need to be there, and reasonably priced.
- tajen 10y ago
  The army gets interested in electric vehicles for troop transport because of their range. It happens that 1. A hybrid can basically provide the same range, 2. A hybrid can regenerate energy when the driver slows down, providing much better range, and 3. They can approach a target in silence for the last 15 minutes, thanks to the electric motor. The drawback is, they struggle with on-field repairs.
esaym 10y ago
You don't need 4 tires to get traction. Just need a 2 cylinder John Deere! https://youtu.be/sZBjNKhcWtY https://youtu.be/sZBjNKhcWtY
slr555 10y ago
All this talk of traction and moving forward is great but one of the most important things to remember about 4WD or AWD especially in snow and ice is that is doesn't enhance your ability to brake. It is easy to feel fearless being able to accelerate on a slippery road but a bummer to discover you can't stop (yes, I get that engine braking plays a role...).
- lathiat 10y ago
  which is precisely why tyre choice is more or less equally important on your little buzz box as it is your high end sports car.
rascul 10y ago
This article falsely assumes certain differential setups based on 4WD or AWD.
repler 10y ago
The newer Jeeps actually have both! It's an improvement over AWD and they call it "Active Drive". It engages/disengages the center differential (4WD) as needed automatically, but can also transfer power to be greater in the front or rear as desired - with presets for 60/40 and 40/60 rear/front power. (Sport mode, Mud/Snow) It's also got traction control w/ ABS, and it uses the brake locking differential technique on each axle as the article describes in advanced AWD systems. Of course they have different variations, one of which also has locking differentials on top of everything else. It's amazing to drive. It's all fully automatic too.
lsllc 10y ago
Alright, time to bust a myth. As an AWD owner, every time I have a tire issue, they tell me I need to replace all 4 even if the others are good ... because AWD. "Won't someone think of the tires ... the tires need to to be the same size so they can rotate at the same rate." Clearly, with AWD the car is designed to allow wheels to rotate at different rates (hence the diffs). Otherwise you would't be able to make a turn. Once you dispel that myth, they then tell you to replace the pair ... Why do dealerships, repair shops and tire places keep insisting on replacing all 4? ... good for tire sales I guess.
- mariusc23 10y ago
  The reasoning I've heard is that your differential will get worn out.
  - lsllc 10y ago
    That makes no sense to me. Unless you are driving in straight lines, the 4 wheels will always be turning at different rates (and consequently, the front and back axles will be rotating at different rates).
userbinator 10y ago
No mention of the "Detroit Locker", IMHO one of the more clever automatic-locking differential designs --- it uses a pair of dog clutches which are normally engaged, one for each side, and the wheel that spins faster is allowed to. It's self-contained and requires no additional controls, just like a regular differential. I think the only reasons why they didn't become more popular are due to their cost and the noise they make --- a very noticeable clunking on tight turns as the outer wheel tries to go faster and unlocks, then locks again as the vehicle straightens out. http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differentials/detroit-locker/index.htm http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differen... Having driven a "4WD" with all three differentials of this type, I now wonder if it would be classified as 4WD or AWD.
- linsomniac 10y ago
  Detroit locker is effective, but I wouldn't really call it clever. For that I think of Torsen, the Torque Sensing differential that uses planetary gears. The problem with the Detroit is primarily noise, as you mention. The problem with Torsen is that it doesn't work in the presence of near 0 traction on one of the outputs, so people will drag the hand brake in that case.
linsomniac 10y ago
I don't really buy this article. The terms AWD and 4WD don't really define a set of functions, but the article says they do then waves hands at all the exceptions. Really, from what I've seen, current usage is AWD is a 4WD system marketed for on road use, and 4WD is an AWD system targeted to off road use. The biggest indicator seems to be if a vehicle has a low range transfer case, it will likely be marketed as 4WD. Also, if it is marketed as 4WD, HOPEFULLY it will have under body armor. Remember, not all AWD/4WD systems are equal. Last winter our neighbor's Subaru Impreza with Symmetric AWD (which Subaru claims is super good) couldn't make it up a small incline by our houses. Both my Audi and Land Cruiser on all season tires had no problem.
- djsumdog 10y ago
  I owned a WRX that I never got stuck in snow. I once came up to a hill with a Sabb and Civic both spinning; and a semi-truck that couldn't really move when they were in the way. I had to wait for both of them to give up, left off my clutch, and crawled up the hill.
Avernar 10y ago
In my Jeep Wrangler, I can instantly tell I forgot to switch from 4WD to 2WD as I turn onto pavement from a dirt road.
pkulak 10y ago
And then there are the dual motor cars like Teslas and Toyotas that have no mechanical link between front and rear axle at all. As you'd imagine, this can allow for a heck of a lot of flexibility.
dan_m2k 10y ago
Great explanation
mtreis86 10y ago
This article is good but goes overboard in explanation. AWD and 4wd by all accounts are just buzz words for having front and rear differentials. In actual use, a 4x4 (a term I mean to refer to both 4x4s and awds as its just a buzz word) has three differentials. A center diff splits power from the transmission out to the other diffs, which split power to the wheels. There are three general types of diffs, whether centered or front or back: Open (just gears, sends power the the least resistant path) Locked (a spindle connects the input directly to the output and all three spin proportionally) Posi (multiple ways of actuating this, can be viscous clutch, electric or air powered actuation, or mecanical geared torsion style - regardless the point is to keep power at all wheels, some with variable speeds allowing for turns - others act like a spindle) Most "awd" cars have a open gear in all three diffs. My jeep has a spindle in the front and center and an air locker in the rear (not the stock design). My old BMW 325ix has a viscous clutch in the center and in the back, and open in the front. My dad's audi has open gears all around and uses the brakes to actuate were the power is delivered. At the end of the day tho, its three differentials.
- sliverstorm 10y ago
  There is a non-buzzword practical difference; one system cannot be driven on dry pavement (part-time 4x4). One system can be driven on dry pavement (full-time AWD).
joushou 10y ago
This definition is broken. You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker). A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories. If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing. Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.
libso 10y ago
AWD should be redefined as Automated Wheel Drive or Auto all Wheel Drive.
outworlder 10y ago
Can't we do away with all the complexity by just having 4 electric motors and a gas engine as a generator? Then each wheel can be computer controlled independently.

404 points by hackuser 10y ago

bitL 10y ago

Great! Finally somebody explained this to me, I thought it was just a marketing term related to different patents of similar ideas. I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I thought they were the same, just different names, so I argued with the ranger that I do in fact have 4WD, which is in my case just called AWD and in other cases 4x4 as well. Well, he was still pretty upset but the "I am a European and we have different cars" finally disarmed him...

japhyr 10y ago
Did they make you turn around? Did your car do fine, or did it get a bit hairy?
- bitL 10y ago
  He actually stopped me after climbing up back to the viewing platform. The car was some US-made SUV with high clearance (don't remember if it was Ford or GM), just there was a sign AWD in the back (I specifically asked for a 4x4 rental vehicle). The road was fine, I think the issue might have been a few streams you had to cross at the bottom of the valley, but SUV had no problem with them. Ranger wanted to fine me but after arguing about 4WD, AWD, 4x4 he dropped it, still a bit angry.
blackguardx 10y ago
Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.
- ghaff 10y ago
  The Waipio Valley road, as I understand it, is mostly paved but extremely steep and narrow/twisty. So the 4x4 requirement is probably more about being able to control speed without burning up brakes and dealing with often wet conditions. However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of saying that, if you just have a random rental car and aren't used to driving very rough and often narrow/exposed roads, this is probably not a good idea.
  - dsr_ 10y ago
    And in particular, the Hawaiian rental car counters will tell you when you rent which roads your car is not allowed to go on.
    - rconti 10y ago
      I've always wondered why this matters. I've got car insurance, I've got additional rental coverage on my credit card, and there are plenty of other vendors from whom to rent cars if you want to blacklist me.
      - Drdrdrq 10y ago
        Well, they still have trouble with you if you get stuck or wreck the car, so it is easier for them to just list the "forbidden" roads. Not sure, but your insurance might be void if you crash the car where you were not supposed to drive.
      - mikeash 10y ago
        They probably don't want you taking a road you shouldn't, getting hurt, and then suing them because they didn't warn you about the dangers. Or they get tired of having all their cars smashed up. Insurance covers it, but it's still annoying.
      - bitL 10y ago
        Having violated this all the time I've been to Hawai'i I can understand why do they do that - for example, if you continue on the road to Hāna/Maui beyond, down to the southern side of Haleakalā the road becomes really dodgy, later turns into gravel and there is no water/gas for xy miles available, so for casual American tourists it might be pretty scary. Also, driving the north-western part of Maui is on a very narrow road over cliffs and fallen rocks (not as bad as parts of Tenerife though). I actually had an encounter on a pretty tall cliff where I was driving Subaru Forrester and some US female celebrity named Kim was driving the opposite way in even a larger car; we drove past each other with smiles in about 1mph to avoid collision and she must have been right on the edge with her car while I was almost scratching the right side rocks. Even Saddle Road between Maunas on the Big Island could be pretty demanding to casual tourists in fog and really slippery when descending down to Hilo in strong rain, which is like always (saw a crash once). There are also some scary roads on Kaua'i and O'ahu. In US it's so much easier to get a driving license than in EU, so your casual drivers are not used to fast and safe driving like in Germany, many off-road segments like in Spain or navigating chaotic situations as in 3rd world countries, and your society is generally on the safe side (even excessively so).
wil421 10y ago
AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road. 4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so. I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road". Cars don't have proper ground clearance.
- waterphone 10y ago
  > The downside is you shouldn't use it over 10mph or so. That's not strictly true, and varies between implementations/transfer case designs. In Jeep Wranglers, for example, which have both high and low gearing options in 4WD, the owner's manual will indicate that you can drive at any speed (on loose surfaces such as sand or dirt) in 4HI and 0–25 MPH in 4LO.
  - wil421 10y ago
    You right that's why I said 10 or so. Try taking a turn at 25 in 4LO. I'm not sure of the use case for 25 in LO. My Grand Cherokee can operate the same way but I doubt I would go into LO at those speeds. It also will switch to 4HI when it detects wheel slip. A friend of mine had a old truck that would randomly switch to 4LO on the highway. It's pretty scary driving 60 and having 4LO switch on.
    - brianwawok 10y ago
      In dirt taking the turn at 25 is fine. The inside wheel will just spin a little.
      - refurb 10y ago
        Outside wheel wants to travel more than the drivetrain so it would slip, not spin correct? The inside wheel would spin if the outside doesn't slip.
to3m 10y ago
This distinction isn't really understood in the UK either, at least not as far as I've ever seen. If your car drives all 4 wheels, it has 4 wheel drive - no, seriously! - and you just have to know what its limitations might be. AWD vs 4WD seems to be one of those distinctions that Americans have, like drink vs beverage, that British English lacks completely.

japhyr 10y ago

Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. This provides great traction, but a vehicle locked in 4WD cannot safely be operated on dry pavement because its front and rear axles are forced to rotate at the same speeds." I remember being scared when I bought my first 4wd vehicle, a '91 Toyota pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and I was worried about going from snowy roads to dry pavement. When I moved on to an Outback (AWD, not 4wd) I wondered why I didn't have to worry about transitioning between slippery roads and dry pavement. Now I understand. There's also a great video from 1937(!) that shows why we need differentials, and how they work. Here's a direct link to the video if you're going to skip the article: https://www.youtube.com/watch?v=yYAw79386WI https://www.youtube.com/watch?v=yYAw79386WI

neom 10y ago
woooow! Re: youtube video - Go to market in 1930s was actually how to go to market. When you think that Chevrolet really got going in 1920, 17 years later their marketing was super on point (here we are still watching it today, content marketing at it's best).
sunstone 10y ago
And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes more sense.
- Mikeb85 10y ago
  Well both tactics work in different scenarios. When power goes to the slipping wheels, it helps keep the car drive strait. For example, in the rain or light snow. If one wheel slips, then suddenly power goes to the wheel with traction, you'll veer off-course. When it goes to the wheels with traction, it helps get the car un-stuck. This is usually what you want when you're doing serious off-roading, stuck in snow, etc... Also, in this scenario you're usually going very slowly/not moving, so the sudden change of direction won't be a problem. Modern AWD does a bit of both - it'll send power to wheels with traction, but will also compensate by sending power to a wheel on the other side of the car. For example, if there's no traction on your left front tire, it'll send power to your right front tire and left rear tire, and reduce power on your right rear tire to keep you going strait.

dghughes 10y ago

>Torque goes to whichever of the four wheels has the least grip. That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere. In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4WD and AWD?

singlow 10y ago
He is talking about what happens when there is no lock. He goes on the clarify what happens when there are locks or viscous couplings.
rwmurrayVT 10y ago
>To counteract this, the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. The differential contains a nifty little clutch. When the vehicle detects no traction, it engages and sends the torque to the correct location. Otherwise you'd end up in a situation similar to engaged cruise control with no traction. Limited slip differentials and locked differentials cause much confusion.
johngalt 10y ago
Mechanically a drivetrain has no means of differentiating between a tire that is slipping vs a tire at that's on the outside of a turn. A 4wd car with three open differentials will absolutely send all of the engine power to one slipping wheel. The author is describing the tricks used to get around that fact. Namely locking differentials or limited slip differentials. The end result is a fraction of the torque makes it to other Wheels depending on the design of the drivetrain. The author spent a lot of time on locking differentials when those are rare. Limited slip differentials are more common. Most awd cars will have a LSD in the center diff. There are torsen diffs which do what you describe but they are rare.
- kinofcain 10y ago
  Even a torsen diff, or similar helical/gear type limited slip diffs are "torque multiplying", they need a load on one wheel in order to send the "multiplied" load to the wheel with traction. Modern stability/traction systems equipped with helical diffs will apply the brake to the "free" wheel in order to provide a load to the mechanical diff. You can also apply the brakes yourself if you have a torsen diff but no electronics. A clutch-based diff or some other type of locking diff would otherwise be required to solve the "wheel in the air" problem without applying an artificial load to the free wheel.

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spacerace 10y ago

Jesus what a SHIT article It gets so many things wrong I don't even know where to start. Why is this garbage spam on Hacker News?

rwmurrayVT 10y ago
No it doesn't. I think you're just confused on fundamental mechanics.
- spacerace 10y ago
  That's rich. The lack of understanding of the fundamental mechanics is exactly what makes this article shit and your comment along with it. But do go ahead a school me on this PLEASE.
  - phpnode 10y ago
    We're all here to learn, if you've got some insight into what the article got wrong then why don't you school us?
  - spacerace 10y ago
    To user: phpnode that replied to me. I can't reply directly to you but that's a fair enough point. I was avoiding doing so before because I didn't (and still don't) think this article is even deserving of being picked apart. I would be happy to do so for the sole benefit of users here though. Just give me a little time to get back from doing some errands and I promise I'll clarify my grievances with the information in the article.
    - phpnode 10y ago
      Still waiting for you to make good on this promise :)
  - sctb 10y ago
    These kinds of comments are not OK on Hacker News. Please stop. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html

kinofcain 10y ago

There's a lot of blurring and different labeling of 4WD systems based on the market segment. Generally it's more helpful to think of 4WD/AWD as fitting into three categories: 1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be welded and they'll turn at the same speed. 2: A system with a center differential, in which both axles are driven but may turn at different speeds (Almost always labeled AWD). The differential may allow full or progressive locking and/or torque vectoring. It's rare to find one of these that can fully lock (Subaru WRX STI is the only one I can think of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or that can partially lock in response to slipping of one axle (often with the brand name Torsen). These are almost always found on passenger cars, and with few exceptions they're found on cars with longitudinally mounted engines (Mitsubishi EVO being the only transverse layout car with a center diff I can think of), that means they're usually found on higher end cars that are based on front-engine rear-wheel drive models. 3: A system with a power-take-off unit (often brand name Haldex), in which one axle is driven and a clutch pack can partially or fully engage the other axle. (this is often labeled AWD when put on a passenger car, and 4WD when placed on a "truck") The axles can operate at different speeds but can also fully lock in the way a transfer case can. Typically you find these on "AWD" cars that are based on transverse-mounted front-drive models as it's easier to add such a system and get partial engagement of the rear axle, but you'll also find them on performance AWD applications based on rear-drive mid/rear-engine models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford Raptor. These tend to operate more like an automatic transfer case, as the only way to partially engage the secondary axle is to slip the clutch, which you don't want to do 100% of the time. (Ford actually calls this system a transfer case on the Raptor). You'll see all sorts of spurious claims about systems "sending 100% of the torque to the rear axle" (often just means that the system can fully lock, and 100% torque to the rears means the fronts are in the air). You'll also see things like "rear biased", which sometimes means you have a center differential that overdrives the rear axle by default, which is great, but is often misleadingly applied to PTO/Haldex systems where the rears are also overdriven, but only during the rare times when the system is engaged. Basically if you're looking for traction in low speed settings where you might not have grip at all on one axle, like offroading or being stuck in the snow, you want as many of your axles locked as possible, which technically you can get with any of these systems, but is usually seen on transfer cases or haldex/PTO systems. If you're driving quickly on pavement with some traction, you usually want something more like a center differential, preferably one that can partially or entirely lock when you lose some grip at one end.

ebbv 10y ago

One thing that this article only touches on briefly is that more and more AWD systems today are just open differentials plus the TCS instead of using more expensive and heavier limited slip differentials. These modern systems are lighter and cheaper but they have disadvantages; not least of which is that using the brakes to force power to move around wastes torque, and it's only going to be as good as the sensors. If the ABS/TCS system has any issues your AWD goes out too. I've owned cars with every type of AWD system (Audi, Subaru and currently a Honda) and day to day they all work just as well. Given that, I suppose the manufacturers are ultimately right to go with the cheaper and weight saving system.

dsfyu404ed 10y ago
The brake based system is great if the traction control is programmed for "get me up this snowbank" and not "prevent me from being an idiot" There was an episode of Dirt Every Day where Fred and co. took a Jeep Renegade(?), or whichever the small one is, off-roading there were some great scenes of the traction control in action. Basically by spinning the dial to rocks the traction control knows to use ABS to approximate how one would work the brakes with a torsen.
chiph 10y ago
The Honda VTM-4 system is quite good in it's intended operating environment - which is operation during winter weather. It uses electromagnetic clutches in the rear differential housing to direct torque to the rear wheels with left/right independence. This gives you the effect of a locking differential, up to the friction limit of the clutch material. There's no center differential in the system, and the front differential is open (but Honda includes a TCS, which is cheap because once you have the ABS sensors & pump, it's just software). Even during good weather on dry roads, the system has benefits by engaging the clutches when accelerating from a stop, so all four wheels are driven when you need to get going. The clutches are disengaged after about 20 mph so you get the fuel economy of a FWD vehicle.

mschuster91 10y ago

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trailer in trucks. And a fuel-electric drivetrain could easily plug in a battery or supercaps for braking energy recuperation, and the fuel engine could always run at peak efficiency/exhaust gas condition...

kinofcain 10y ago
The diesel-electric/hydraulic systems are used in situations where the gearing ratio would have to be absurdly high, they're not used because they're efficient or because they have good torque vectoring capabilities. The electric motor can produce nearly 100% of torque at zero RPM, which helps locomotives get several million pounds of train moving. In your car, truck, or even semi, first gear plus some clutch/torque converter slippage is plenty of mechanical advantage to get going, and once you're underway, a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor. There's also the matter of power distribution, if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.
- 4ad 10y ago
  > a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor. I doubt it, generators and motors operate at 99% efficiency. Do you have any reference? > if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel. That assumes that engines are so small that they operate at 100% under normal conditions. This doesn't have to be the case, electric engines are so small, you can have bigger-sized engines where one single engine can use all the power. The nature of electric engine is such that the loss of efficiency by doing this is minimal. Plus you need to do this only fractions of the time, so the engine doesn't have to be designed to operate in high power mode at full duty cycle.

Animats 10y ago

There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range. It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well documented, but Tesla cars aren't intended for off-roading. The most advanced systems are seen on locomotives. Modern heavy locomotives (GE Evolution series) use three-phase AC synchronous motors driven by IGBT inverters from power from a Diesel generator. Each axle has its own motor. All axles are normally locked together electronically by the control software, so no wheel can slip ahead of the others. Locomotives can be cabled together so that the axles on multiple locomotives synchronize. Now that's all-wheel drive. This is a huge win when starting a heavy freight train. Locomotive axles that are slipping provide little pulling power and damage the tires and rails.

hackuser 10y ago
> Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Is that different than the traction control used by other automakers? > Tesla's all wheel drive system has a separate motor for the front and rear wheels. That is very cool. And it sounds much more complex - I wonder if the complexity is worth the benefit.
- CarVac 10y ago
  Separate motors for front and rear wheels is far less complex than adding more moving parts. For AWD, you need a driveshaft and a differential. The differential has a lot of parts, there are a lot of bearings that sap power, and it adds a lot of weight. Using separate electric motors for front and rear instead increases the available power, enables unlimited torque ratios from all-forward to all-rear, and has many fewer moving parts.
  - 10y ago

dsfyu404ed 10y ago

The performance of either system is completely dominated by the traction aid at the axles. The article is otherwise good because most AWD vs 4WD stuff completely igores the axles (because it's a heck of a lot to explain if you really want to compare the various systems) but kinda misses the main point. Open, bad traction control (cuts throttle, applies a little brake), good traction control (all brake no throttle), the many varieties of LSDs, automatic lockers, manual lockers, Lincoln lockers and spools all have a huge affect on traction. Lincoln lock the diffs of any AWD crossover, screw with some wiring so it's full time and you'll have something that can out wheel a $50k Jeep but trades off a tiny bit of grocery getting capability.

rsync 10y ago
Thank you for introducing me to the concept of lincoln-locking a differential. Appreciated.

ynniv 10y ago

Of course, this only matters when you are driving four wheels with one engine. Now that we're going electric you can have one engine per wheel, or one front engine and one back engine and use ABS across single axles. It's kind of amazing how much progress we have made with simple internal combustion engines.

schwede 10y ago

The video from the 30s was fascinating. It gave a perfect progression of complexity while explaining the concept of a differential. I'd like to see more of those kind of videos (from now our from clear back then).

auganov 10y ago
You might like this one https://www.youtube.com/watch?v=mBz30JhtKkg https://www.youtube.com/watch?v=mBz30JhtKkg The channel also has some more.
LyndsySimon 10y ago
You might like Sheaffer's "The Twenty-Six Old Characters", c. 1947. It touches a bit on the progression of language, but it's really about writing instruments. It culminates in explaining Sheaffer's "Snorkel" mechanism, which was probably the most novel and complex pen system ever designed. I have several and love them :) https://youtu.be/1xUDehNvbrE https://youtu.be/1xUDehNvbrE

ledriveby 10y ago

Subaru's higher end AWD system has separate output shafts from the engine for the front and the rear, with electronically-controlled clutches modifying the torque. Default is 60/40 front biased, but it can send 100% of the torque to either end of the car. Excellent performance with no manual mode switching needed, also manages good fuel economy at the same time. Best system in the business.

bluedino 10y ago
However, if you don't have locking differentials you can still get into the situation where one wheel slips and the wheel that grips doesn't get any power - as shown by the Subaru getting stuck on the hill in this video: https://youtu.be/hrdb_UVTa20?t=543 https://youtu.be/hrdb_UVTa20?t=543
- deleted 10y ago
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- post_break 10y ago
  There is a trick that seems idiotic but if you have a mechanical hand brake you can stop the wheel from free spinning. I'm not sure how well it would work for a front wheel, maybe you could hold the brake slightly while on the throttle. Again, seems ass backwards but if you think about what you're doing you're basically "locking" the open diff.
abstrakraft 10y ago
Not quite. All AWD Subarus (i.e. except the BRZ) have one output shaft out of the engine, 3 out of the transmission/transaxle (2 to the front tires, 1 to the rear diff), and 2 out of the rear diff. Torque split varies between models and years with most near the 60/40 you quote, but the only one (in the US, anyway) that can send 100% torque to either end is the STI with DCCD on full lock.
pps43 10y ago
To add to confusion, Subaru sells several very different systems and calls all of them the same "all time all-wheel drive". There is viscous coupling center differential that goes with manual transmissions and nearly always splits torque 50/50 between front and rear, computer-controlled limited-slip clutch pack that comes with most automatics and changes torque split with speed, and some newer systems that do even more.

kilovoltaire 10y ago

Wow, early cars connected the engine to only one wheel, to completely avoid issues of differential. (According to the cool old video linked in the article.)

mod 10y ago
My cousin had a firebird, I think a '67, that (seemingly?) had 1wd. He could do donuts that left one trail. Not sure if by design or some kind of failure...

giardini 10y ago

4WD/AWD are almost never necessary in USA and Europe today, to the extent that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk (for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in bad weather. But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and maintenance are significantly higher: often you must "fiddle" with them, woe to you should you damage them due to bad judgement. They reduce mileage, increase car weight, and reduce reliability. I prefer a RWD car for most driving, since it handles better and I'm never climbing bare hills or plunging through swamps, but I was raised on oversteering go-karts. "All Wheel Drive Does Not Make You Safer": http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-does-not-make-you-safer/ http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-do... "The Myth of the All-Powerful All-Wheel Drive:" http://www.popularmechanics.com/cars/a3091/the-myth-of-the-all-powerful-all-wheel-drive-15202862/ http://www.popularmechanics.com/cars/a3091/the-myth-of-the-a... 4WD/AWD safety - Is 4WD/AWD safer on snow and ice? http://www.4x4abc.com/jeep101/safe.html http://www.4x4abc.com/jeep101/safe.html

landryraccoon 10y ago
In California you are legally required to either have chains equipped or a 4WD vehicle with snow tires under R1 and R2 snow conditions: http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm It doesn't matter what the studies say, 4WD is legally recognized as safer than 2WD in California.
- giardini 10y ago
  AFAICT you're required to have the chains available, so they're usually in the trunk, unless you're in a situation where they're required. Of course if you buy chains you should immediately put them on your vehicle, if only to ensure that later, when you need them, they will fit. Once you've done that, then you can put them in the trunk. I disagree with your interpretation of the law: dot.ca.gov simply state what they require and say nothing about which is superior. And note that 4WD/FWD vehicles _also_ must carry chains in the R2 & R3 areas (from the URL you posted http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm): "(NOTE: Four wheel/all wheel drive vehicles must carry traction devices in chain control areas.)" Unfortunately for an AWD/4WD vehicle, following this provision costs twice as much as for a FWD car and will take twice the time to install (not an insignificant factor in freezing rain) and, should you fail to install on both front and rear properly, or should they suffer mechanical failure, may put one in a bad situation. Sometimes it's better to do less.

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tzs 10y ago

The problem of the outer wheel going farther during a turn also occurs with trains, and trains do not have differentials to let the wheels have different angular velocity. On a train, the inner and outer wheels have the same angular velocity regardless of turns. How trains accomplish this is interesting: http://jalopnik.com/5820296/how-trains-make-turns-without-differentials http://jalopnik.com/5820296/how-trains-make-turns-without-di... I'm linking to Jalopnik, which embeds Richard Feynman's video on train turning, rather than directly to the video because there are some interesting comments on the site. In particular, see the comment from railroad employee "Gonemad" about how they have a system that automatically greases the wheels before turns, which seems rather counterintuitive at first.

justinph 10y ago

In real life, in most situations, ground clearance, good tires, traction control and ABS make more difference than driving all four wheels, most of the time. Personally, I'm more concerned with stopping than starting, and that's where ABS and good tires matter and what kind of transmission you have doesn't. This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with different vehicles on rollers: https://www.youtube.com/watch?v=9cuZYTQLfA0 https://www.youtube.com/watch?v=9cuZYTQLfA0 I spent a lot of time looking at this several years ago when my family was planning on taking a vacation to the Outer Banks of North Carolina where you have to drive on the sand. We ended up renting a Ford Explorer that had a "sand mode" in it's electronically controlled awd setup. Worked great.

analog31 10y ago
Something I've learned is that snow tires make a big difference during the winter, and I've read that they make a bigger difference than AWD.
- justinph 10y ago
  Absolutely. Have owned a Prius and a Subaru. The Prius with good snow tires was phenomenal in the snow, better than the Subaru with all-seasons.
- paganel 10y ago
  Have been a driver for only 2 years but I can confirm that. I live in Eastern Europe where we tend to have at least one or two snow storms per winter, and, indeed, having winter tires and ABS makes a ton of difference, even on a 1.4-liter FWD vehicle like mine. And, even more important, you have to keep a level head when you see that there's snow on the road and not try to do anything crazy, like making sudden left or right wheel movements. Also try to keep a safe distance from the vehicle in front of you, and, before you venture on a trip outside of the city, just check the weather forecasts: if there's a "red code" storm announced you'd better postpone your journey, only a tank can get you out from something like this: https://www.youtube.com/watch?v=Frf2lV77fl0 https://www.youtube.com/watch?v=Frf2lV77fl0

examancer 10y ago

Kind of an odd post to see on hacker news. As somewhat of a car buff my take away is that this article gives an adequate high level explanation but leaves out a lot of useful details, especially to anyone evaluating 4WD/AWD vehicles, like the tremendous differences between modern AWD systems.

elihu 10y ago

It'll be interesting to see what happens when off-road vehicles start becoming electrified. Separate front/rear motors eliminate the need for that middle differential or transfer case, and if we eventually transition to a separate motor for each wheel, we'll be rid of all of the differentials and have much better ground clearance as well as better traction. For now, I suppose electric trucks/jeeps are more affected by "range anxiety" than daily-driver sedans, since people tend to imagine themselves driving to some remote location when they buy them. But as technology improves, I would love to see something like a modern reincarnation of a 1960's era CJ-5 (but with better safety features) -- simple, easy to modify, nothing that doesn't need to be there, and reasonably priced.

tajen 10y ago
The army gets interested in electric vehicles for troop transport because of their range. It happens that 1. A hybrid can basically provide the same range, 2. A hybrid can regenerate energy when the driver slows down, providing much better range, and 3. They can approach a target in silence for the last 15 minutes, thanks to the electric motor. The drawback is, they struggle with on-field repairs.

esaym 10y ago

You don't need 4 tires to get traction. Just need a 2 cylinder John Deere! https://youtu.be/sZBjNKhcWtY https://youtu.be/sZBjNKhcWtY

slr555 10y ago

All this talk of traction and moving forward is great but one of the most important things to remember about 4WD or AWD especially in snow and ice is that is doesn't enhance your ability to brake. It is easy to feel fearless being able to accelerate on a slippery road but a bummer to discover you can't stop (yes, I get that engine braking plays a role...).

lathiat 10y ago
which is precisely why tyre choice is more or less equally important on your little buzz box as it is your high end sports car.

rascul 10y ago

This article falsely assumes certain differential setups based on 4WD or AWD.

repler 10y ago

The newer Jeeps actually have both! It's an improvement over AWD and they call it "Active Drive". It engages/disengages the center differential (4WD) as needed automatically, but can also transfer power to be greater in the front or rear as desired - with presets for 60/40 and 40/60 rear/front power. (Sport mode, Mud/Snow) It's also got traction control w/ ABS, and it uses the brake locking differential technique on each axle as the article describes in advanced AWD systems. Of course they have different variations, one of which also has locking differentials on top of everything else. It's amazing to drive. It's all fully automatic too.

lsllc 10y ago

Alright, time to bust a myth. As an AWD owner, every time I have a tire issue, they tell me I need to replace all 4 even if the others are good ... because AWD. "Won't someone think of the tires ... the tires need to to be the same size so they can rotate at the same rate." Clearly, with AWD the car is designed to allow wheels to rotate at different rates (hence the diffs). Otherwise you would't be able to make a turn. Once you dispel that myth, they then tell you to replace the pair ... Why do dealerships, repair shops and tire places keep insisting on replacing all 4? ... good for tire sales I guess.

mariusc23 10y ago
The reasoning I've heard is that your differential will get worn out.
- lsllc 10y ago
  That makes no sense to me. Unless you are driving in straight lines, the 4 wheels will always be turning at different rates (and consequently, the front and back axles will be rotating at different rates).

userbinator 10y ago

No mention of the "Detroit Locker", IMHO one of the more clever automatic-locking differential designs --- it uses a pair of dog clutches which are normally engaged, one for each side, and the wheel that spins faster is allowed to. It's self-contained and requires no additional controls, just like a regular differential. I think the only reasons why they didn't become more popular are due to their cost and the noise they make --- a very noticeable clunking on tight turns as the outer wheel tries to go faster and unlocks, then locks again as the vehicle straightens out. http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differentials/detroit-locker/index.htm http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differen... Having driven a "4WD" with all three differentials of this type, I now wonder if it would be classified as 4WD or AWD.

linsomniac 10y ago
Detroit locker is effective, but I wouldn't really call it clever. For that I think of Torsen, the Torque Sensing differential that uses planetary gears. The problem with the Detroit is primarily noise, as you mention. The problem with Torsen is that it doesn't work in the presence of near 0 traction on one of the outputs, so people will drag the hand brake in that case.

linsomniac 10y ago

I don't really buy this article. The terms AWD and 4WD don't really define a set of functions, but the article says they do then waves hands at all the exceptions. Really, from what I've seen, current usage is AWD is a 4WD system marketed for on road use, and 4WD is an AWD system targeted to off road use. The biggest indicator seems to be if a vehicle has a low range transfer case, it will likely be marketed as 4WD. Also, if it is marketed as 4WD, HOPEFULLY it will have under body armor. Remember, not all AWD/4WD systems are equal. Last winter our neighbor's Subaru Impreza with Symmetric AWD (which Subaru claims is super good) couldn't make it up a small incline by our houses. Both my Audi and Land Cruiser on all season tires had no problem.

djsumdog 10y ago
I owned a WRX that I never got stuck in snow. I once came up to a hill with a Sabb and Civic both spinning; and a semi-truck that couldn't really move when they were in the way. I had to wait for both of them to give up, left off my clutch, and crawled up the hill.

Avernar 10y ago

In my Jeep Wrangler, I can instantly tell I forgot to switch from 4WD to 2WD as I turn onto pavement from a dirt road.

pkulak 10y ago

And then there are the dual motor cars like Teslas and Toyotas that have no mechanical link between front and rear axle at all. As you'd imagine, this can allow for a heck of a lot of flexibility.

dan_m2k 10y ago

Great explanation

mtreis86 10y ago

This article is good but goes overboard in explanation. AWD and 4wd by all accounts are just buzz words for having front and rear differentials. In actual use, a 4x4 (a term I mean to refer to both 4x4s and awds as its just a buzz word) has three differentials. A center diff splits power from the transmission out to the other diffs, which split power to the wheels. There are three general types of diffs, whether centered or front or back: Open (just gears, sends power the the least resistant path) Locked (a spindle connects the input directly to the output and all three spin proportionally) Posi (multiple ways of actuating this, can be viscous clutch, electric or air powered actuation, or mecanical geared torsion style - regardless the point is to keep power at all wheels, some with variable speeds allowing for turns - others act like a spindle) Most "awd" cars have a open gear in all three diffs. My jeep has a spindle in the front and center and an air locker in the rear (not the stock design). My old BMW 325ix has a viscous clutch in the center and in the back, and open in the front. My dad's audi has open gears all around and uses the brakes to actuate were the power is delivered. At the end of the day tho, its three differentials.

sliverstorm 10y ago
There is a non-buzzword practical difference; one system cannot be driven on dry pavement (part-time 4x4). One system can be driven on dry pavement (full-time AWD).

joushou 10y ago

This definition is broken. You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker). A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories. If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing. Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.

libso 10y ago

AWD should be redefined as Automated Wheel Drive or Auto all Wheel Drive.

outworlder 10y ago

Can't we do away with all the complexity by just having 4 electric motors and a gas engine as a generator? Then each wheel can be computer controlled independently.